Faults in electric power systems occur for a variety of reasons, such as trees or cranes coming into contact with power lines, transformer failure, shorts in load circuits, and so forth. Once the line section experiencing the fault has been identified, sectionalizing switches are used to isolate the faulted line segment in order to bring the non-faulted sections back into service. Detecting and locating faults on a power system is therefore essential for public safety as well as minimizing the extent of outages caused by line faults and providing high quality service to the users of electric power. Line mounted overcurrent fault indicators commonly utilized on distribution circuits in urban environments identify faults by detecting high fault currents that significantly exceed normal load currents. These work quite well when there is a solidly grounded, low impedance fault that causes the line currents to increase by 2 to 10 times the load current.
While overcurrent protection devices are effective at detecting the high fault currents caused by low impedance faults, it is much more difficult to detect high impedance faults in which the fault currents are in the approximate range of normal load currents. For these types of high impedance faults, the normal load currents mask the fault currents preventing the overcurrent line monitors from detecting the faults. Techniques have been developed for detecting high impedance faults using three phase voltages and currents, but these systems are costly and typically do not have sufficient distance resolution to effectively determine fault location.
While three phase current monitors are relatively inexpensive, it is generally not economically feasible to install three phase voltage monitors at various points along distribution lines where the high impedance faults usually occur. Because three phase voltage are required, high impedance fault detection systems are typically installed only at substations where these measurements are readily available. This results in an entire radial distribution line, from the substation to the end of the power line, typically being taken out of service until the high impedance fault has been cleared. Of course, in most cases the fault actually occurs far from the substation and a large portion of the radial power line between the substation and the fault could remain in service if fault detection and isolation systems capable of detecting high impedance faults were located at various points along the distribution line. But at present this is not economically feasible due to the high cost of installing three phase voltage monitoring equipment at various points along distribution lines.
Techniques have been developed for detecting high impedance faults by identifying signatures created by arcing faults. But these techniques require complex algorithms and are prone to false detection caused by parasitic harmonic content on power lines. In addition, their reliability can be suspect because they require batteries at line voltage which are difficult to replace and have finite lifetimes.
There is also a misconception that sensitive ground protection typically used to detect low ground current will detect high impedance faults. But in reality, unbalanced loads limit the sensitivity of ground protection. Moreover, a down conductor can result in relatively balanced fault loads and reduced neutral current.
This can easily mislead the protection equipment into failing to detect a high impedance fault. Significant safety issues can result, for example when a tree or crane comes into contact into a power line and the people in the immediate surrounding area are in grave danger. The conventional overcurrent protection devices have great difficulty in detecting a fault of this type due to the high impedance, low current nature of the fault. As a result, the time duration between occurrence of the fault and its detection by the system operators can easily be minutes to days. In some cases, the fault may not be identified until panic 911 calls have been received from the general public notifying the utility company to turn off the power.
There is, therefore, a continuing need for improved and more cost effective electric power fault isolation systems for high impedance faults. There is, in particular, a need for a high impedance fault isolation system that does not require voltage measurements. There is a further need for a high impedance fault isolation system that does not require battery powered controller.